This invention relates to electrochemical storage cells, and, more particularly, to the preparation of electrodes for a Ni/MHx cell and preparation of the cell itself.
Rechargeable electrochemical storage cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge as useful power. Familiar examples of the rechargeable cell are the lead-acid cell used in automobiles and the nickel-cadmium cell used in various portable electronic devices. Other types of cells having a greater storage capacity for their weight and volume include those based upon the reduction and oxidation of nickel oxide at a cathode, and the corresponding oxidation and reduction of hydrogen at an anode. Such cells are desirably used in weight-critical, long-life applications such as the batteries in spacecraft. One familiar cell based upon this electrochemistry is the nickel oxide/pressurized hydrogen cell.
Another type of cell under development is the nickel/metal hydride cell (also known generically in the art as the "Ni/MHx cell"), which has the advantage that a pressurized container is not required because the anode reaction product is a solid rather than a gas. At the anode of the nickel/metal hydride cell, a reversible electrode reduction reaction of water at the surface of a metal alloy (the "active material") produces a solid metal hydride and hydroxide ion. The metal hydride has, in general, a different volume than the corresponding metal. The anode is therefore subjected to volumetrically induced strains during charging/discharging cycles. If the anode is not properly designed and fabricated, these strains may lead to a premature failure. When the cell is utilized in a spacecraft application requiring many charging/discharging cycles and is not readily accessible for repairs, such failures can have significant adverse consequences.
It is known to fabricated the anode of a Ni/MHx electrochemical cell by mixing the finely divided hydride-forming metal powder with finely divided carbon and a polymer. The mixture is then heated to soften the polymer, and the mixture is forced into the anode substrate structure. Upon cooling, the polymer binds the metal powder and the carbon to the anode substrate. This technique has the major disadvantage that a portion of the surface area of the metal particles is blocked from access by the electrolyte. Some portion of the metal powders can be fully encapsulated by the polymer and become inaccessible for the electrochemical reaction, thereby reducing the utilization of the anode active material. Additionally, the polymer may not fully wet the anode substrate, resulting in long-term debonding of the active material and partial or total failure of the anode. Also, the polymer may not be sufficiently resilient to accommodate the volumetric changes during long-term cycling, leading to disintegration of the particle-to-particle bonding of the alloy within the anode structure.
The above discussion has focused on the anode, but there may be comparable issues associated with the cathode. Thus, there is a need for an improved approach to fabricating electrodes (anodes and cathodes) and the corresponding cell for a Ni/MHx electrochemical storage cell and other storage cells of this general type. The electrodes should be highly efficient, and also be resistant to long-term damage, as from volumetrically induced strains. The electrodes must also be stable during long-term exposure in the electrolyte. The electrodes are also desirably inexpensive to produce. The present invention fulfills this need, and further provides related advantages.